Delete the declarations' stream once every node runs the new node-engine, and assert it no more (issue 1945, B13)

A node away during the switch finds its last declaration on the stream; once every node's have names the
node-engine registered, no node reads one, so the stream and the nodes' consumers on it go, said once, and a raise
leaves them out (ADR 0539 clause 11).

cmd/mesh-controller/composed_start_test.go

@@ -91,3 +91,22 @@ func TestTheReplacedControllerIsItsNewestEarlierBuild(t *testing.T) {
 		t.Errorf("a controller with no earlier build replaced one")
 	}
 }
+
+// 1945/AC14 (unit half): the declarations' stream stays while one node's have names another node-engine than the one
+// registered, or none, and goes once every node's names it.
+func TestTheDeclarationsGoOnlyOnceEveryNodeRunsTheNewEngine(t *testing.T) {
+	n6 := sdkdelivery.SourcePrefix2 + strings.Repeat("6", 64)
+	nodes := []string{"ace", "g14", "novox"}
+	if declarationsDone(nodes, map[string]string{"ace": n6, "novox": n6}, n6) {
+		t.Errorf("done while g14 says nothing (it sleeps)")
+	}
+	if declarationsDone(nodes, map[string]string{"ace": n6, "g14": "src2:old", "novox": n6}, n6) {
+		t.Errorf("done while g14 runs the old node-engine")
+	}
+	if !declarationsDone(nodes, map[string]string{"ace": n6, "g14": n6, "novox": n6}, n6) {
+		t.Errorf("not done with every node on the new node-engine")
+	}
+	if declarationsDone(nodes, map[string]string{"ace": "", "g14": "", "novox": ""}, "") || declarationsDone(nil, nil, n6) {
+		t.Errorf("done with no fingerprint registered, or no node")
+	}
+}